300sd tranny

Hi, I have an '83 300sd, which has suddenly developed a probem engaging 1st gear. The fluid was low and brown. I figure I should change the fluid and filters, but is there anything else I can check while I have the pan off? It races for a few seconds while trying to engage, then engages with some slip and shudder then goes through the rest of the gears fine. btw it has always had a very minimal clunk when it engaged 1st gear, and it also never would kickdown into first either. Thanks in advance